Method of forming a FET having ultra-low on-resistance and low gate charge

ABSTRACT

In accordance with an exemplary embodiment of the invention, a substrate of a first conductivity type silicon is provided. A substrate cap region of the first conductivity type silicon is formed such that a junction is formed between the substrate cap region and the substrate. A body region of a second conductivity type silicon is formed such that a junction is formed between the body region and the substrate cap region. A trench extending through at least the body region is then formed. A source region of the first conductivity type is then formed in an upper portion of the body region. An out-diffusion region of the first conductivity type is formed in a lower portion of the body region as a result of one or more temperature cycles such that a spacing between the source region and the out-diffusion region defines a channel length of the field effect transistor.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.10/754,276, filed Jan. 8, 2004, which is a divisional of U.S.application Ser. No. 09/640,955, filed Aug. 16, 2000, now U.S. Pat. No.6,696,726, entitled “Vertical MOSFET with Ultra-low Resistance and LowGate Charge”, which disclosures are incorporated herein by reference intheir entirety.

Two other related patents are U.S. Pat. No. 6,437,386, entitled “Methodfor Creating Thick Oxide on the Bottom Surface of a Trench Structure inSilicon” and U.S. Pat. No. 6,444,528, entitled “Selective OxideDeposition in the Bottom of a Trench,” both of which are assigned to thepresent assignee and are incorporated herein by reference for allpurposes.

BACKGROUND OF THE INVENTION

The present invention relates to field effect transistors (FETs) and, inparticular, to trench metal-oxide-semiconductor (MOS) transistors andmethods of fabricating the same.

Power Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs) arewell known in the semiconductor industry. One type of MOSFET is adouble-diffused trench MOSFET, or what is known as a “trench DMOS”transistor. A cross-sectional view of a portion of a typical n-channeltrench DMOS transistor 10 is shown in FIG. 1. It should be pointed outthat the relative thickness of the various layers is not necessarilydrawn to scale.

The trench DMOS transistor 10, shown in FIG. 1, includes an n-typesubstrate 100. An n-type epitaxial layer 102 is formed over substrate100, and a p-type body region 108 is formed in epitaxial layer 102through an implant/diffusion process. One or more trenches 109 extendthrough body region 108 and into region 102 a of epitaxial layer 102.Gate oxide layer 104 lines the sidewalls and bottom of each trench 109and a conductive material 106, typically doped polysilicon, lines gateoxide layer 104 and fills each trench 109. N+ source regions 110 flankeach trench 109 and extend a predetermined distance into body region108. Heavy body regions 112 are positioned within body region 108,between source regions 110, and extend a predetermined distance intobody region 108. During the high temperature cycles of the process(e.g., the anneal steps for activating the dopants in body region 108,source regions 110, and heavily doped body regions 112) the n-typedopants in substrate 100 tend to diffuse into epitaxial layer 102 thusforming the substrate out-diffusion region 101. Finally, dielectric caps114 cover the filled trenches 109 and also partially cover sourceregions 110. Note that trench DMOS transistor 10 also typically includesone or more metal layers, which contact source regions 110, withadjacent metal layers separated by an insulating material. These metallayers are not shown in FIG. 1.

FIG. 2 shows a doping concentration profile, taken along a cross-sectionlabeled “xx” in FIG. 1. Cross section xx is representative of theresistance path 116 that a drain-to-source current, I_(DS), encountersas charge carriers travel from source region 110 to the drain of trenchDMOS transistor 10, when trench DMOS transistor is on. The variousregions that comprise path 116 are source region 110, body region 108,portion 102 a of epitaxial layer 102, substrate out-diffusion region 101and substrate 100.

The resistance encountered by I_(DS) due to the presence of thesevarious regions is typically quantified as the drain-to-sourceresistance, R_(DS)(on). A high R_(DS)(on) limits certain performancecharacteristics of the transistor. For example, both thetransconductance, g_(m), of the device, which is a measure of thecurrent carrying capability of the device (given a certain gate voltage)and the frequency response of the device, which characterizes the speedof the device, are reduced for higher R_(DS)(on). Another factor thatlimits the speed of the trench DMOS transistor is the gate oxide charge,Q_(g). The higher Q_(g) is the larger the gate-to-drain overlapcapacitance becomes and, consequently, the lower the switchingcapability of the device becomes.

Because the drain-source voltage is dropped almost entirely across thechannel region, which comprises the body and epitaxial layers, thechannel length, channel resistance and channel concentration profile arecritical characteristics that affect the operating performance of atrench MOSFET. Whereas the absolute values of these characteristics areimportant, so too is the controllability of their variation. Widedevice-to-device variations negatively affect the reproducibility of adevice having desired performance capabilities.

BRIEF SUMMARY OF THE INVENTION

In accordance with the present invention, a trench DMOS transistor ischaracterized by an ultra-low on resistance R_(DS)(on) and a low gatecharge. The structure and method of manufacturing the DMOS transistorminimizes variations in the transistor characteristics by controllingsubstrate out-diffusion.

In accordance with an embodiment of the invention, a field effecttransistor is manufactured as follows. A substrate cap region of a firstconductivity type silicon is formed such that the substrate cap regionforms a junction with a substrate of the first conductivity typesilicon. A body region of a second conductivity type silicon is formedsuch that the body region forms a junction with the substrate capregion. A trench extending through at least the body region is formed. Asource region of the first conductivity type is formed in the bodyregion, wherein during one or more temperature cycles, dopants of thefirst conductivity type out-diffuse into a lower portion of the bodyregion to form an out-diffusion region of the first conductivity type inthe lower portion of the body region such that a spacing between thesource region and the substrate out-diffusion region defines a length ofa channel region of the field effect transistor.

In one embodiment, the trench further extends through the out-diffusionregion and the substrate cap region, and the conductive material extendsthrough a substantial depth of the out-diffusion region.

In another embodiment, the dielectric material is thicker along thebottom of the trench than along its sidewalls.

In another embodiment, the out-diffusion region extends from aninterface between the body region and the substrate cap region into thebody region.

In another embodiment, during the one or more temperature cycles thesubstrate cap region influences said out-diffusion of the dopants of thefirst conductivity type into the body region such that the length of thechannel region varies less and thus is substantially predictable.

In one embodiment, the substrate cap region has a thickness of less thanor equal to two micrometers.

In accordance with another embodiment of the invention, a field effecttransistor includes a substrate of a first conductivity type silicon. Asubstrate cap region of the first conductivity type silicon forms ajunction with the substrate. A body region of a second conductivity typesilicon forms a junction with the substrate cap region. A trench extendsat least through the body region. A source region of the firstconductivity type is in an upper portion of the body region. Anout-diffusion region of the first conductivity type is in a lowerportion of the body region such that a spacing between the source regionand the out-diffusion region defines a channel length of the fieldeffect transistor.

In one embodiment, the out-diffusion region extends from an interfacebetween the body region and the substrate cap region into the bodyregion.

In another embodiment, the substrate cap region and the body region areepitaxial layers.

In another embodiment, the substrate cap region has a thickness of lessthan or equal to two micrometers.

A further understanding of the nature and advantages of the inventionmay be realized by reference to the remaining portions of thespecification and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of a conventional trench DMOStransistor;

FIG. 2 shows a doping concentration profile along a cross-section “xx”of the trench DMOS transistor shown in FIG. 1;

FIG. 3 shows a cross-sectional view of an exemplary n-channel trenchDMOS transistor 30 according to one embodiment of the present invention;

FIG. 4 shows an exemplary doping concentration profile, taken along across-section “yy” of the trench DMOS transistor shown in FIG. 3;

FIG. 5 shows an exemplary process flow, according to another aspect ofthe invention, for fabricating the trench DMOS transistor shown in FIG.3; and

FIGS. 6A-6J show cross-sectional views of the formation of the trenchDMOS transistor according to the process flow shown in FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a trench MOSFET device, and itsmethod of manufacture, that can be used in applications such as cellularphone power supplies and battery switching. The trench MOSFET of thepresent invention is defined by a structure having a low drain-to-sourceresistance, low gate charge and a method of fabrication that minimizesdevice-to-device variations in operating characteristics by controllingout-diffusion from the transistor substrate.

FIG. 3 shows a cross-sectional illustration of an exemplary n-channeltrench DMOS transistor 30 according to one embodiment of the presentinvention. Trench DMOS transistor 30 includes an n-type substrate 300,which has a resistivity of, for example, 1-5 mΩ-cm, over which asubstrate cap region 301 is formed. Substrate cap region 301 is heavilydoped and has a resistivity of, for example, 1 mΩ-cm. Substrate capregion 301 functions to provide a more constant resistivity range thanwhat substrate vendors typically guarantee. For example, substratevendors typically guarantee that the resistivity of an Arsenic n-typesubstrate be only somewhere within the range of 1-5 mΩ-cm. As explainedbelow, the more precisely controlled resistivity of substrate cap region301, relative to substrate resistivities, ensures a more predictable andstable channel length.

A p-type body region 308 is epitaxially formed over substrate cap region301. The thickness and resistivity of p-type body region 308 are, forexample, 4 μm and 0.1 Ω-cm, respectively. One or more trenches 309extend through body region 308, substrate cap region 301 and,preferably, a portion of substrate 300. Gate oxide layer 304 lines thesidewalls and bottom of each trench 309 and a conductive material 306,for example, doped polysilicon, lines gate oxide layer 304 and fillseach trench 309. The thickness of gate oxide layer 304 is preferablythicker at the bottom of each trench 309 than on the sidewalls of thetrench 309.

N+ source regions 310 flank each trench 309 and extend a predetermineddistance into body region 308. Heavy body regions 312 are positionedwithin body region 308, between source regions 310, and extend apredetermined distance into body region 308. Dielectric caps 314 coverthe filled trenches 309 and also partially cover source regions 310.

An n-type substrate out-diffusion region 302 extends up from theinterface between body region 308 and substrate cap region 301 into bodyregion 308. Substrate out diffusion region 302 is formed as a result ofn-type dopants in substrate cap region 301 out-diffusing into bodyregion 308 during high temperature cycles such as the oxide layerformation and anneals to activate the dopants in source regions 310 andheavily doped body regions 312.

Because the resistivity of cap layer 301 varies far less than that ofsubstrate 300, the extent to which substrate out-diffusion region 302extends into body 308 can be predicted more accurately. Since channellength 318 of DMOS 30 is defined by the spacing between source 310 andsubstrate out-diffusion 302, the improved predictability andcontrollability of the out-diffusion of region 302 enables tightercontrol over and the reduction of channel length 308. Better control ofchannel length 308 leads to a more predictable and reproducibleR_(DS)(on), Q_(g) and breakdown voltage.

Trench DMOS transistor 30 also includes one or more metal layers, whichcontact source regions 310, with adjacent metal layers separated by aninsulating material. These metal layers are not shown in FIG. 3.

Comparing trench DMOS transistor 30 to trench DMOS transistor 10 in FIG.1 reveals some important distinctions. First, as was described above, itis preferred that the thickness of gate oxide layer 304 be larger at thebottoms of each trench 309 than on the sidewalls of each trench 309. Thereason for this is that a thicker gate oxide at the bottom of trenches309 alleviates high electric fields in the vicinity of the bottom oftrenches 309, thereby providing a higher breakdown voltage, BVdss. Therelatively greater thickness also has the effect of reducing thegate-drain overlap capacitance, so that the gate charge, Q_(g), isreduced.

Second, trench DMOS transistor 30 does not incorporate an n-typeepitaxial layer as trench DMOS transistor 10 does (see, layer 102 inFIG. 1). The primary purpose of the n-type epitaxial layer is to providea region for depletion to avoid reach through. However, while notnecessarily limited to, the trench DMOS transistor of the presentinvention is envisioned to be mainly for low voltage applications. Abenefit of the absence of any n-type epitaxial layer in trench DMOStransistor 30 is that a reduced current path is realized so thatR_(DS)(on) is lowered. As explained above, a lower R_(DS)(on) improvescertain performance capabilities of the device, which are characterizedby, for example, a higher transconductance, g_(m), and an improvedfrequency response.

Finally, body region 308 is formed by epitaxial deposition, as comparedto an implant/diffusion process used in forming body region 108 in thetrench DMOS transistor shown in FIG. 1. The diffusion step in themanufacture of a trench DMOS is typically performed at high temperatureand operates to drive all junctions, including substrate out-diffusionregion 101 in trench DMOS transistor 10 shown in FIG. 1. A typicaldiffusion cycle used in the manufacture of trench DMOS transistor 10 ofFIG. 1 can result in a substrate out-diffusion region thickness of over2 μm. Because a diffusion cycle is not required for forming body region308 of trench DMOS transistor 30, the thickness of substrateout-diffusion region 302 can be made much thinner, for example, lessthan or equal to 1 μm. Moreover, for a given channel length, channel 318can hold more charge than that of a conventional trench DMOS transistorhaving a body region formed using an implant/diffusion process. Becausechannel 318 of trench DMOS transistor 30 can hold more charge, it isless likely that drain-to-source punch-through will occur. Hence,channel length 318 of trench DMOS 30 can be reduced. The reduction inchannel length 318 and the thickness of substrate out-diffusion region302 result in lower R_(DS)(on).

Referring now to FIG. 4, there is shown an exemplary dopingconcentration profile taken along a cross-section labeled “yy” fortrench DMOS transistor 30 shown in FIG. 3. Comparing this doping profileto the doping profile in FIG. 2 of the conventional trench DMOStransistor 10 in FIG. 1, shows that (1) no n-type epitaxial layer isused in the trench DMOS transistor 30 of the present invention; (2)channel length 318 of DMOS transistor 30 of the present invention isshorter; and (3) substrate out-diffusion region 302 of DMOS transistor30 is thinner and has a steeper concentration gradient than that ofconventional DMOS transistor 10. All of these characteristics have theeffect of reducing the overall drain to source current path, therebymaking R_(DS)(on) smaller.

Referring now to FIG. 5, there is shown an exemplary process flow,according to another aspect of the invention, for fabricating a trenchDMOS transistor. This process flow can be used, for example, tofabricate the trench DMOS transistor shown in FIG. 3. The process flowshown in FIG. 5 will now be described in reference to FIGS. 6A through6J.

Initially, a substrate 300, having a resistivity of, for example 1 to 5mΩ-cm is provided in step 500. This is shown in FIG. 6A. Next, in step502, a substrate cap region 301 is epitaxially formed over substrate300. Substrate cap region 301 has a resistivity of, for example lessthan or approximately equal to 1 mΩ-cm and a thickness less than orequal to 2 μm. In one embodiment, substrate cap region 301 has athickness of approximately 1 μm. The structure corresponding to step 502is shown in FIG. 6B.

In step 504, a p-type body region 308 is epitaxially formed oversubstrate cap region 301. In one embodiment, body region 308 has a depthof approximately 4 μm, and a resistivity of about 0.1 mΩ-cm. Thestructure corresponding to step 504 is shown in FIG. 6C. Next, in step506, an initial oxide layer is formed over the p-type body region 308,over which an active area of transistor 30 is defined using conventionalphotolithography techniques.

After the active area has been defined, trenches 309 are formed in step508. This step is shown in FIG. 6D. An anisotropic etch may be used tocreate trenches 309. The anisotropic etch is in the form of a plasma,which is an almost neutral mixture of energetic molecules, ions andelectrons that have been excited in a radio-frequency electric field.Different gases are used depending on the material to be etched. Theprincipal consideration is that the reaction products must be volatile.For etching silicon, the reactants may be, for example, He:O₂, NF₃ andHBr the pressure may be, for example, 140 mTorr and the duration of theetch may be approximately 3 minutes. In this example, the trenches havea depth of approximately 2.5 μm. As shown in FIG. 6D, each trench 309extends vertically downward from an exposed surface of body region 308,into and through body region 308, through substrate out-diffusion layer302, through substrate cap region 301 and partially into substrate 300.

Next, an oxide plug 303 is formed at the bottom of each trench 309 asshown in FIG. 6E. These oxide plugs 303 can be formed in a variety ofways. Two embodiments are shown in FIG. 5 by steps 510-516 and steps520-522. In the embodiment of steps 510-516, in step 510,sub-atmospheric chemical vapor deposition (SA-CVD) is used to depositoxide on the sidewalls, bottom and over the upper and lower corners ofeach trench 309. Then, in step 512, the oxide is etched back so thatonly an oxide plug 303 remains at the bottom of each trench 309. Asacrificial oxide, having a thickness of about 500 Å may then bedeposited (step 514) and then stripped (step 516) to prepare the trenchsidewalls for a gate oxide. The sacrificial oxide and strip steps areoptional.

Oxide plug 303 can be alternatively formed using a process known ashigh-density plasma chemical vapor deposition (HDP-CVD) as shown by theembodiment of steps 520-522. In step 520, oxide is deposited on thesidewalls, bottom and over the upper and lower corners of each trench309. Then, in step 522, the oxide is etched back using a wet etch toleave an oxide plug 303 at the bottom of each trench 309.

Next, in step 524, a gate oxide 304 is formed on the sidewalls oftrenches 309 as shown in FIG. 6F. The thickness of gate oxide 304 inthis example, is about 200 Å. Next, in step 526, trenches 309 are linedand at least partially filled with polysilicon 306 and then doped using,for example, an n-type implant or by administering a conventional POCL₃doping process. Doping can also be performed using an in-situ process,i.e., as the polysilicon is deposited. The structure corresponding tostep 528 is shown in FIG. 6G. Next, in optional step 528, the thresholdvoltage of the structure can be adjusted by administering a p-typeimplant having, for example, an energy and dose of 70 keV and3×10¹³/cm², respectively.

Next, in another optional step 530, p+ heavy body regions 312 can beformed between adjacent trenches 309 as shown in FIG. 6H. This isaccomplished by defining the surface areas through which heavy bodyregions 312 are to be formed using, for example, conventionalphotolithography. Through the defined surface areas, two separate p-type(e.g., boron) implants are performed, although in some applications asingle implant may be sufficient. In this example, a first implant isperformed at a dose and energy of, for example, 2×10¹⁵/cm² and 135 keV,respectively and a second implant is performed at a dose and energy of5×10¹⁴/cm² and 70 keV, respectively. The primary purpose of the firstimplant is to bring the depth of heavy body regions 312 as deep as isnecessary to compensate for the n+ source regions which are formed laterin the process. The second implant has a low energy but a high dose. Thepurpose of this implant is to extend high concentration of the p+ heavybody from the first implant to the surface so that an ohmic contact canbe formed. The dose is made high enough to accomplish this but not sohigh as to overcompensate the n+ source region. In an alternativeembodiment, heavy body region 312 can be formed following a contactdefining step (step 536), which is performed later in the process.

In step 532 source regions 310 are formed as shown in FIG. 6I. Similarto formation of heavy body region 312, in this example a double implantmay be used. In this example, surface areas through which source regions310 are to be formed are defined using, for example, conventionalphotolithography. Through these surface areas, two separate n-typeimplants are performed, although in some applications a single implantmay be sufficient. In this example, a first implant of arsenic isperformed at a dose and energy of, for example, 8×10¹⁵ cm² and 80 keV,respectively and a second implant of phosphorous is performed at a doseand energy of 5×10¹⁵/cm² and 60 keV, respectively. The purpose of thefirst implant is to form source regions 310 and the purpose of thesecond implant is to extend source regions 310 to the surface so that asource contact can be formed.

Whereas the above description described formation of heavy body regions312 prior to the formation of source regions 310, in an alternativeembodiment heavy body regions 312 could be formed following formation ofsource regions 310.

Next, in step 534, an insulating layer, e.g., borophosphosilicate glass,having a thickness in the range of about 5 to 15 kÅ is deposited overthe exposed surface of the entire structure. Then the insulating layeris densified or “flowed”.

In step 536, the insulating layer is patterned and etched using, forexample, standard photolithography, to define electrical contact areasfor the trench DMOS structure. As shown in FIG. 6J, the dielectric etchis controlled to preserve insulating caps 314 over trenches 309. Afterstep 536, metallization and passivation steps are performed, althoughthey are not shown in the process diagramed in FIGS. 5 and 6. Oneskilled in the art would understand, however, what is necessary toperform these steps.

In the above process flow, the temperature cycles associated withformation of dielectric layers (steps 506 and 524) results in theout-diffusion of the n-type dopants from substrate cap region 301 intobody region 308, thus forming substrate out-diffusion region 302 in bodyregion 308 as shown in FIGS. 6D through 6F. A further out-diffusion ofdopants into body region 308 occurs during the temperature cyclesassociated with formation of heavy body regions 312 (step 530) andsource regions 310 (step 532). The thickness and dopingconcentration/gradient of substrate out-diffusion region 302 areprimarily determined by the characteristics of substrate cap region 301(e.g., its doping concentration and thickness), and the total process DT(the product of diffusion coefficient and time). Therefore, to minimizethe extent of out-diffusion of region 302 into body region 308, RTP(rapid thermal processing) can be used everywhere possible in theprocess flow to minimize the overall DT.

Although the invention has been described in terms of specific processesand structures, it will be obvious to those skilled in the art that manymodifications and alterations may be made to the disclosed embodimentwithout departing from the invention. For example, an alternative toepitaxially forming substrate cap region 301 is to form substrate capregion 301 within substrate 300 by implanting dopants and driving thedopants into substrate 300. As another example, an alternative toepitaxially forming body region 308 a is to initially form an n-typeepitaxial layer over substrate cap region 301 followed by implantingp-type dopants and driving the dopants into the n-type epitaxial layersuch that the body region is formed within the epitaxial layer. Thisparticular variation is advantageous in that: (i) it allows integrationof the transistor cell structure of the invention with a wider varietyof termination structures, and (ii) a body region formed by diffusion(as opposed to an epitaxially formed body region) leads to lessvariations in threshold voltage. As yet another example, a p-channeltrench DMOS may be formed by using silicon layers with complementaryconductivity types relative to those of the trench DMOS structure shownin FIG. 3. Also, all of the values provided such as for dimensions,temperatures, and doping concentrations, are for illustrative purposesonly and may be varied to refine and/or enhance particular performancecharacteristics of the trench DMOS transistor. Hence, thesemodifications and alterations are intended to be within the spirit andscope of the invention as defined by the appended claims.

1. A method of forming a field effect transistor, comprising:epitaxially forming a substrate cap region of a first conductivity typesilicon over and in contact with a substrate of the first conductivitytype silicon; epitaxially forming a body region of a second conductivitytype silicon over and in contact with the substrate cap region; forminga plurality of trenches each extending at least through the body region;lining the sidewalls and bottom of each trench with a dielectricmaterial; at least partially filling each trench with a conductivematerial; and forming a plurality of source regions of the firstconductivity type in an upper portion of the body region, wherein duringone or more temperature cycles, dopants of the first conductivity typein the substrate cap region out-diffuse into a lower portion of the bodyregion to form an out-diffusion region of the first conductivity typeextending from an interface between the body region and the substratecap region into the body region such that a spacing between the sourceregions and the out-diffusion region defines a length of a channelregion of the field effect transistor.
 2. The method of claim 1 wherein:each trench extends through the out-diffusion region, and the conductivematerial in each trench extends through a substantial depth of theout-diffusion region.
 3. The method of claim 1 wherein during the one ormore temperature cycles the substrate cap region influences saidout-diffusion of the dopants of the first conductivity type into thebody region such that the length of the channel region varies less andis thus substantially predictable.
 4. The method of claim 1 wherein thesubstrate cap region has a thickness less than or equal to 2 μm.
 5. Themethod of claim 1 wherein the out-diffusion region has a graded dopingconcentration decreasing in a direction away from an interface betweenthe out-diffusion region and the substrate cap region.
 6. A method offorming a field effect transistor, comprising: providing a substrate ofa first conductivity type silicon; forming a substrate cap region of thefirst conductivity type silicon such that a junction is formed betweenthe substrate cap region and the substrate; forming an epitaxial layerof the first conductivity type silicon over the substrate cap region;implanting dopants of a second conductivity type into the epitaxiallayer and driving the dopants into the epitaxial layer to thereby form abody region having a junction with the substrate cap region; forming aplurality of trenches each extending at least through the body region;lining the sidewalls and bottom of each trench with a dielectricmaterial; at least partially filling each trench with a conductivematerial; and forming a plurality of source regions of the firstconductivity type in an upper portion of the body region, wherein duringone or more temperature cycles, dopants of the first conductivity typein the substrate cap region out-diffuse into a lower portion of the bodyregion to form an out-diffusion region of the first conductivity typeextending from the junction between the body region and the substratecap region into the body region such that a spacing between the sourceregions and the out-diffusion region defines a length of a channelregion of the field effect transistor.
 7. The method of claim 6 whereinthe substrate cap region is epitaxially formed over and in contact withthe substrate.
 8. The method of claim 6 wherein the substrate cap regionis formed in the substrate by implanting dopants of the firstconductivity type into the substrate.
 9. A field effect transistor,comprising: a substrate of a first conductivity type silicon; anepitaxial substrate cap region of the first conductivity type siliconover and in contact with the substrate; an epitaxial body region of asecond conductivity type over and in contact with the substrate capregion; a plurality of trenches each extending at least through the bodyregion; a dielectric material lining the sidewalls and bottom of eachtrench; a conductive material at least partially filling each trench; aplurality of source regions of the first conductivity type in an upperportion of the body region; and an out-diffusion region of the firstconductivity type extending from an interface between the body regionand the substrate cap region into the body region such that a spacingbetween each source region and the out-diffusion region defines a lengthof a channel region of the field effect transistor, the channel regionextending vertically along a sidewall of each trench.
 10. The fieldeffect transistor of claim 9 wherein: each trench extends through theout-diffusion region, and the conductive material in each trench extendsthrough a substantial depth of the out-diffusion region.
 11. The fieldeffect transistor of claim 9 wherein the dielectric material in eachtrench is thicker along the bottom of each trench than along thesidewalls of each trench.
 12. The field effect transistor of claim 9wherein the out-diffusion region has a graded doping concentrationdecreasing in a direction away from an interface between theout-diffusion region and the substrate cap region.
 13. The field effecttransistor of claim 9 wherein the substrate cap region has a thicknessof less than or equal to two micrometers.